General relativity and quantum theory are the two pillars of modern physics, peerlessly accurate in their respective realms of the very large and the very small. But where they meet, they produce contradictory answers – and other problems they create mean they provide a far from complete picture.
For more on the basics of the theories, take a look at Reality guide: The essential laws of cosmology and Reality guide: The essential laws of quantum physics.
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PROBLEM 1: Dark matter
Galaxies rotate too quickly for their visible matter
Earth whirls around the sun at a speed determined by its distance, the sun’s mass and gravity’s strength, which is a universal constant of nature. You don’t need general relativity to work that out: Newton’s old-school gravitation will do. The same law should apply to distant galaxies swirling around their common centre of mass.
In the 1930s, however, the astronomer Fritz Zwicky discovered that the outer parts of the Coma galactic cluster were rotating far faster than the cluster’s estimated mass allowed. In the 1970s, Vera Rubin confirmed the huge mismatch in a clutch of spiral galaxies similar to the Milky Way. She estimated they must contain about six parts of invisible matter for every one part of visible matter.
This “dark matter” must interact gravitationally to produce the motion, but hardly at all through the other forces of nature. The standard model of particle physics provides no particle that fits the bill, and efforts to detect dark matter particles beyond the standard model, or manufacture them in high-energy particle collisions, have so far come to naught. Something is missing.
Reality guide: How everything fits together
The six principles that rule the universe… and the six big problems we still can’t crack
Neutrinos
Could neutrinos be dark matter? The standard model of particle physics says these elusive particles have no mass, but experiments now say they do have a small one – the only direct contradiction of a standard model prediction so far. But the extra mass seems unlikely to be enough to explain dark matter, unless as-yet undiscovered new varieties of “sterile” neutrino exist. Recent results from the European Space Agency’s Planck satellite and the IceCube Neutrino Observatory in Antarctica seem to discount that.
PROBLEM 2: Dark energy
The universe is flying apart faster and faster
In the late 1990s, two groups studying far-off supernovae discovered that these stellar explosions were consistently fainter than expected. Their conclusion: the space their light had travelled through to get to us had stretched more than expected, so the supernovae were further away than supposed.
Dark energy is the name for whatever is causing this accelerating expansion. It dominates the cosmos, making up by the latest reckoning some 68 per cent of everything there is. But what is it? Perhaps a vacuum energy of the sort that quantum particles might create by popping in and out of free space. This would be a resurrection of the cosmological constant that Einstein originally introduced into the equations of general relativity, and then dropped. Or perhaps a “quintessence”, an as yet undiscovered fifth force of nature.
Both identities have their problems, and there could be another way out. A universe with a variable density of matter would expand at different rates in different places, possibly producing an illusion of accelerated expansion. So if we drop the cosmological principle we might possibly get rid of dark energy, too.
The cosmological constant
When Einstein created his static universe model, he added an extra term in the equations of general relativity to counteract gravity’s pull. He later called this cosmological constant his “greatest blunder”. Tweaked to represent a quantum-mechanical energy of free space, it might explain dark energy – only quantum theory supplies a huge 10120 times as much as is needed to set the universe speeding on its way. This is perhaps the most glaring numerical mismatch in all of physics.
PROBLEM 3: Inflation
Faster-than-light expansion spawns many other universes
Range your eye across the cosmos, and a couple of features are hard to explain. It is darn near geometrically “flat”, and even far-off bits all have roughly the same temperature.
Cosmic inflation solves these problems at a stroke. In its earliest instants, the universe expanded faster than light (light’s speed limit only applies to things within the universe). That ironed out wrinkles in its early chaotic self and meant that even now far-flung parts were once in close contact, so could swap heat.
In 2014, researchers claimed to have seen ripples from inflation imprinted on the cosmic microwave background. But this proved mistaken, and it’s not clear what would have made the early universe inflate anyway. Worse, inflation is very difficult to stop, creating a multiverse of causally disconnected universes that eternally bud off from one another.
One way out might be to weaken the constant speed of light. If the speed of light was faster in the early universe, that would also explain the temperature problem. Perhaps light is still slowing now, just at a rate that is imperceptible even to our most sensitive detectors.
PROBLEM 4: Force unification
Our theories of reality don’t get along
The standard model of particle physics covers three forces of nature, but doesn’t unify the electroweak and strong theories neatly. Gravity, meanwhile, stands apart as the only force we can’t describe with quantum theory. Any effort to make it quantum – to describe it through the exchange of particles called gravitons, rather than through general relativity’s space-time warpings – is ripped apart by uncontrollable infinities that render all calculations meaningless.
When subatomic particles interact, gravity is generally so weak that it can safely be ignored. But in some realms the two must come together: in black holes, for example, or in describing the universe’s tiny origins in the big bang. Without a quantum theory of gravity – a first step to a unifying “theory of everything” – science faces an impenetrable barrier to ultimate enlightenment.
Just by the by, any theory of quantum gravity will require breaking the link between gravitational and inertial mass embodied by the equivalence principle – undermining a foundation stone of modern physics.
Black holes
Black holes are super-dense objects that swallow everything, including light, that strays too near. They come in different sizes: supermassive black holes lurk at the heart of most galaxies, and stellar-mass ones form when spent stars collapse in on themselves. Predicted by general relativity, black holes are also places where gravity is so strong that it can no longer be neglected on quantum theory’s small scales – so we currently have no understanding of what happens at the edge of a black hole or inside one.
PROBLEM 5: Fine-tuning
We can’t explain the numbers that rule the universe
The standard model of particle physics is highly effective, but incomplete. It doesn’t fully explain the different strengths of weak, strong and electromagnetic forces, for example, or the masses of the particles it introduces. These quantities have to be measured experimentally and tacked on to the theory. Were any of them to have even marginally different values, the universe would look very different. The Higgs boson’s mass, for example, is just about the smallest it can be without the universe’s matter becoming unstable.
Similar “fine-tuning” problems bedevil cosmology. Why does the amount of dark energy and dark matter in the universe seem so finely balanced as to allow galaxies to form? Why is the carbon atom structured so precisely as to allow enough carbon for life to exist in the universe? And perhaps the most finely balanced of all: why is the cosmos have lots of matter, and no antimatter?
Antimatter
Antimatter exists. Unwittingly predicted by Paul Dirac in 1928, it was discovered in cosmic rays a little later. The mystery is rather why there is so little of it – or indeed anything at all. The standard model says matter and antimatter should have formed in equal quantities in the big bang, “annihilating” in a puff of light shortly after. Some currently inexplicable tiny imbalance caused matter to win out – allowing our universe of stars and galaxies to come to be.
PROBLEM 6: The measurement problem
Do we inadvertently control everything that happens?
Features such as wave-particle duality and entanglement highlight a mystery at the heart of quantum physics. Our decision to measure quantum objects seems to change their nature, forcing them to “collapse” and adopt definite states. So are we co-conspirators in creating reality? If so, what does measuring a quantum object do to it, if anything?
One daring suggestion is the “many worlds” hypothesis, that the universe splits into a multiverse of possibilities every time we measure something. Or perhaps collapse is a preprogrammed, random feature of the quantum world, a principle just as central as wave-particle duality or uncertainty. Or perhaps, as Einstein believed, there’s something fundamentally wrong in how we see quantum theory – in which case we need to review all the principles that underlie it.
For every problem there’s a solution – and we’re not lacking in untested ideas that might propel us to a deeper understanding. To read more about them, see Reality guide: Six radical ideas to change physics.
